1. Field of the Invention
The present invention relates to a light-emitting device including multiple pixels each having a light-emitting element and a unit for supplying a current to the light-emitting element and a method for driving the light-emitting device.
2. Description of the Related Art
Since a light-emitting element emits light by itself, the light-emitting element is highly visible and does not require a back light to be required for a liquid crystal display device. Therefore, a light-emitting element is suitable for thin devices. Furthermore, the viewing angle of a light-emitting device is not limited. Because of these advantages, a light-emitting device having a light-emitting element has recently gathered attentions as an alternative display device to a CRT and/or an LCD.
The intensity of a light-emitting element herein can be controlled by a current or a voltage. The light-emitting element includes an organic light-emitting diode (OLED) and an MIM type electron source element (electron emitting element) to be used for a field emission display (FED).
An OLED (organic light-emitting diode) is a kind of light-emitting element and includes a layer (called electric field light-emitting layer hereinafter) containing an electric field light-emitting material for obtaining electroluminescence generated by applying an electric field thereto, an anode and a cathode. The electric field light-emitting layer is provided between the anode and the cathode and further contains one or multiple layers. These layers may contain an inorganic compound. The electroluminescence in an electroluminescence layer includes light emission (fluorescence) when a singlet exciting state returns to a ground state and a light emission (phosphorescence) when a triplet exciting state to a ground state.
Next, the construction of a pixel of a general light-emitting device and the drive will be described briefly. A pixel shown in FIG. 19 has TFTs 80 and 81, a storage capacitor 82 and a light-emitting element 83.
A gate of the TFT 80 is connected to a scantine 85. One of a source or a drain of the TFT 80 is connected to a signal line 84, and the other is connected to a gate of the TFT 81. A source of the TFT 81 is connected to a terminal 86. A drain of the TFT 81 is connected to the anode of the light-emitting element 83. The cathode of the light-emitting element 83 is connected to a terminal 87. The storage capacitor 82 is provided for retaining a voltage between the gate and source of the TFT 81. Predetermined amounts of voltages are applied from a power source to the terminals 86 and 87, which have a voltage difference.
The term “voltage” herein means a potential difference from the ground unless especially mentioned.
When the TFT 80 is turned on by the voltage of the scanline 85, the voltage of a video signal input to the signal line 84 is input to the gate of the TFT 81. In accordance with the voltage of the input video signal, the gate voltage (a voltage difference between the gate and source) of the TFT 81 is determined. The gate voltage causes the drain current to flow in the TFT 81, and the drain current is supplied to the light-emitting element 83. The light-emitting element 83 emits light by using the supplied current.
A TFT containing polysilicon has a higher field effect mobility than that of a TFT containing amorphous silicon and has a large On-current. Therefore, the TFT of polysilicon can be a more suitable transistor for a light-emitting device. However, the electric characteristic of the TFT of polysilicon is still not compared with the characteristic of a MOS transistor on a single crystalline silicon substrate. For example, the field effect mobility is 1/10 of that of the single crystalline silicon or lower. The TFT of polysilicon easily has variations in characteristics due to a defect in a crystal grain boundary. This is a disadvantage.
When the characteristics such as threshold values and On-currents of the TFT 81 differ in the pixels shown in FIG. 19, the magnitude of the drain current of the TFT 81 differs among pixels even with the same voltage of video signals. As a result, the intensity of the light-emitting element 83 varies.
In order to avoid these problems, various kinds of pixel constructions have been proposed which can suppress the variation in a current flowing though the light-emitting element due to the variations in characteristics of TFTs. The constructions of a current-input pixel and a threshold-value-corrected, voltage-input pixel, which are typical pixels, will be illustrated, and the driving will be further described.
First of all, the construction of the current-input pixel disclosed in Patent Document 1 mentioned below will be described with reference to FIG. 20A.
[Patent Document 1]
JP-A-2001-147659
A pixel in FIG. 20A has TFTs 11, 12, 13 and 14, a storage capacitor 15, and a light-emitting element 16.
The gate of the TFT 11 is connected to a terminal 18. One of the source and drain is connected to a current source 17, and the other is connected to the drain of a TFT 13. The gate of the TFT 12 is connected to a terminal 19. One of the source and drain is connected to the drain of the TFT 13, and the other is connected to the gate of the TFT 13. The gates of the TFT 13 and TFT 14 are connected to each other, and the sources of them are both connected to the terminal 20. The drain of the TFT 14 is connected to the anode of the light-emitting element 16, and the cathode of the light-emitting element 16 is connected to a terminal 21. The storage capacitor 15 holds the voltage between the gate and source of the TFTs 13 and 14. Predetermined amounts of voltages are applied to the terminals 20 and 21, which have a voltage difference.
The TFTs 11 and 12 are turned ON by the voltage to be supplied to the terminals 18 and 19, and the drain current of the TFT 13 is then controlled by the current source 17. Here, since the gate and drain are connected, the TFT 13 operates in the saturated region. The drain current is expressed by Equation 1 mentioned below.
 ID=μC0W/L(VGS−VTH)2/2  [EQ1]
where VGS is a gate voltage, and μ is a mobility. C0 is a gate capacity for a unit area, and W/L is the ratio of a channel width W and a channel length L of a channel forming region. VTH is a threshold value, and ID is a drain current.
In EQ 1, all of μ, C0, W/L, and VTH are fixed values depending on the transistor. Apparently from EQ 1, the drain current of the TFT 13 depends on the gate voltage VGS. Therefore, in accordance with EQ 1, the gate voltage VGS corresponding to the drain current occurs in TFT 13.
Here, since the gates and sources of the TFT 13 and TFT 14 are connected to each other, the gate voltage of the TFT 14 is maintained to be equal to the gate voltage of the TFT 13. Thus, the drain currents of the TFT 13 and TFT 14 are on a proportional basis. When the values of μ, C0, W/L, and VTH thereof are equal, the drain currents of the TFT 13 and TFT 14 are equal. The drain current of the TFT 14 is supplied to the light-emitting element 16, and the light-emitting element 16 emits light with the intensity corresponding to the magnitude of the drain current.
Especially, by increasing the ratio of the On-current of the TFT 14 to that of the TFT 13, a desired magnitude of a current can be supplied to the light-emitting element with a short writing time.
Even after the TFTs 11 and 12 are turned off by the voltage supplied to the terminals 18 and 19, the light-emitting element 16 keeps emitting light as far as the gate voltage VGS of the TFT 14 is retained by the storage capacitor 15.
Next, the construction of the current-input pixel disclosed in Non-Patent Document 1 mentioned below will be described with reference to FIG. 20B.
[Non-Patent Document 1]
Tech. Digest IEDM 98, 875. R. M. A. Dawson etc.
A pixel in FIG. 20B has TFTs 31, 32, 33 and 34, a storage capacitor 35, and a light-emitting element 36.
The gate of the TFT 31 is connected to a terminal 38. One of the source and drain of the TFT 31 is connected to a current source 37, and the other is connected to the source of the TFT 33. The gate of the TFT 34 is connected to a terminal 38. One of the source and drain of the TFT 34 is connected to the gate of the TFT 33, and the other is connected to the drain of the TFT 33. The gate of the TFT 32 is connected to a terminal 39, and one of the source and drain of the TFT 32 is connected to a terminal 40, and the other is connected to the source of the TFT 33. The drain of the TFT 33 is connected to the anode of the light-emitting element 36, and the cathode of the light-emitting element 36 is connected to a terminal 41. The storage capacitor 35 is provided for retaining a voltage between the gate and source of the TFT 33. Predetermined amounts of voltages are applied from the power source to the terminals 40 and 41 and have a voltage difference.
The TFTs 31 and 34 are turned on by the voltage supplied to the terminal 38, and the TFT 32 is turned off by the voltage supplied to the terminal 39. Then, the drain current of the TFT 33 is controlled by the current source 37. Here, since the gate and drain of the TFT 33 are connected, the TFT 33 operates in a saturated region. The drain current is expressed by EQ1. It is understood that the drain current of TFT 33 varies depending on the gate voltage VGS. In accordance with EQ1, the gate voltage VGS corresponding to the drain current occurs in the TFT 33. The gate voltage VGS is retained by the storage capacitor 35.
The drain current flowing in the TFT 33 is supplied to the light-emitting element 36, and the light-emitting element 36 emits light with the intensity corresponding to the magnitude of the drain current.
After the TFTs 31 and 34 are turned off by the voltage supplied to the terminal 38, the TFT 32 is turned on by the voltage supplied to the terminal 39. Here, as far as the gate voltage of the TFT 33 is retained by the storage capacitor 35, the light-emitting element 36 emits light with the same intensity as that when the TFTs 31 and 34 are on.
Even when the characteristics such as threshold values and On-current vary among a current-input pixels shown in FIGS. 20A and 20B, the magnitude of the current to be supplied to the light-emitting element is controlled by the current source. Therefore, the variation in intensity of the light-emitting elements of the pixels can be prevented.
Next, the construction of the threshold value corrected voltage input pixel as disclosed in Patent Document 2 mentioned below will be described with reference to FIG. 21.
[Patent Document 2]
U.S. Pat. No. 6,229,506, Specification
A pixel shown in FIG. 21 has TFTs 51, 52, 53 and 54, storage capacitors 55 and 56 and a light-emitting element 57.
The gate of the TFT 51 is connected to a terminal 59. One of the source and drain of the TFT 51 is connected to a terminal 58, and the other is connected to one electrode of the storage capacitor 55. The other electrode of the storage capacitor 55 is connected to the gate of the TFT 53. The gate of the TFT 52 is connected to a terminal 61. The source of the TFT 52 is connected to the gate of the TFT 53, and the drain of the TFT 52 is connected to the drain of the TFT 53 and the source of the TFT 54. The source of the TFT 53 is connected to a terminal 60, and the storage capacitor 56 is provided for retaining volume between the gate and source of the TFT 53. The gate of the TFT 54 is connected to a terminal 62, and the drain of the TFT 54 is connected to the anode of the light-emitting element 57. The cathode of the light-emitting element 57 is connected to a terminal 63. Predetermined amounts of voltages are applied from the power source to the terminals 60 and 63, which have a voltage difference. Here, the amount of voltage to the terminal 60 is higher than the amount of the voltage to the terminal 63.
First of all, the height of the voltage to be applied to the terminal 58 is equalized to the height of the volume to be applied to the terminal 60. After the voltage to be applied to the terminal 59 is controlled to turn on the TFT 51, the voltages to be applied to the terminals 61 and 62 are controlled to turn on the TFTs 52 and 54. Then, the storage capacitors 55 and 56 start to store charges. When the voltage retained in the storage capacitor 56 exceeds a threshold value (VTH) of the TFT 53, the TFT 53 is turned on.
Next, when the TFT 54 is turned off, the charges stored in the storage capacitors 55 and 56 are discharged through the TFT 53 in ON state. Here, since the TFT 52 is ON, the gate and drain of the TFT 53 are connected. Therefore, the TFT 53 operates in a saturated region. The drain current when charges are discharged is expressed by EQ1.
The discharging continues until ID=0, that is, until the TFT 53 is turned off. Since all of μ, C0, W/L, and VTH are fixed values depending on each transistor in EQ1, VGS=VTH in accordance with EQ1 when ID=0. In other words, when the TFT 53 is turned off by the discharging, the amount of voltage VTH corresponding to the threshold value of the TFT 53 is stored in the storage capacitor 56.
Next, the TFT 52 is turned off, and a voltage VData of a video signal is applied to the terminal 58. Because of the input of the video signal, the sum voltage of the threshold voltage VTH and the voltage VData is stored in the storage capacitor 56 in accordance with the Law of Conservation of Charges.
Next, after the TFT 51 is turned off, the TFT 54 is turned on so that the drain current of the TFT 53 is supplied to the light-emitting element 57. Here, the drain current of the TFT 53 is controlled by the sum voltage of the threshold voltage VTH and the voltage VData retained in the storage capacitor 56. Thus, irrespective of the threshold value VTH of the TFT 53, the current corresponding to the voltage VData is supplied to the light-emitting element 57. Thus, the uneven intensity due to the variation in threshold value can be suppressed.
As described above, the current-input pixel shown in FIG. 20 and the threshold value corrected voltage input pixel shown in FIG. 21 can suppress the variations in a current flowing in the light-emitting element due to the variations in characteristics of TFTs more than the pixel shown in FIG. 19. However, the pixels with those constructions have problems.
Typically as shown in FIG. 20A, a pixel may have two units including a unit (such as the TFT 13) for converting a current supplied to the pixel to a voltage and retaining the voltage and a unit (such as the TFT 14) for passing a predetermined amount of a current corresponding to the retained voltage to the light-emitting element. In this case, the balance of the characteristics between these two units may vary due to a shift of the characteristic of one unit. Then, the amount of a current to be supplied from the driving portion to the light-emitting element cannot be kept in a desired amount. Thus, a variation in intensity of the light-emitting element may occur among pixels.
In particular, when any of μ, C0, W/L, and VTH, which are unique characteristics of a TFT, is shifted in the TFT 13 or TFT 14 in FIG. 20A, the ratio of the drain current of the TFT 14 to the drain current of the TFT 13 may differ among pixels. As a result, the intensity of the light-emitting element may vary among the pixels.
On the other hand, typically as shown in FIG. 20B, a pixel may have a unit (such as the TFT 33) for converting a current supplied to the pixel to a voltage, retaining the voltage, and feeding the amount of a current corresponding to the retained voltage to a light-emitting element. In this case, a current flows in the light-emitting element when the current supplied to the pixels is converted to a voltage. The light-emitting element has a larger capacity. Therefore, when the display changes from a lower grayscale to a higher grayscale, the value of a voltage to be converted from the current is not stable until a certain degree of charges are stored in the capacity of the light-emitting element. Thus, the change in the display from a lower grayscale to a higher grayscale takes time. Conversely, when the display changes from a higher grayscale to a lower grayscale, the value of a voltage to be converted from a current is not stable until extra charges of the capacity of the light-emitting element are discharged. Therefore, the change in the display from a higher grayscale to a lower grayscale takes time.
In particular, in FIG. 20B, when the value of a current supplied from the current source 37 changes, the longer time is required until the gate voltage of the TFT 33 is stable. Thus, the time for writing a current becomes longer. As a result, the persistence of vision may be visually identified, and the advantage of the light-emitting element suitable for moving image display because of the rapid responsibility is not used effectively.
In a pixel typically as shown in FIG. 21, the amount of voltage corresponding to the threshold value of a TFT (such as the TFT 53) for controlling the current to be supplied to a light-emitting element may be written in the storage capacitor in advance. In this case, a current does not flow to the light-emitting element when the amount of voltage corresponding to the threshold value is written into the storage capacitor. Therefore, the writing time does not depend on the capacity of the light-emitting element as shown in FIG. 20B.
However, after charges enough to turn on the TFT 53 are stored in the storage capacitor, the charges must be discharged through the TFT 53 until the voltage retained in the storage capacitor reaches the threshold value voltage VTH. Therefore, the time required for the discharging does not reduce the writing time. Then, the pixels typically as shown in FIG. 21 cannot handle the case that the writing time must be further reduced for the time-grayscale display using digital signals, for example. As a result, the persistence of view may be visually identified in moving image display, and the advantage of the light-emitting element suitable for moving image display because of the rapid responsibility is not used effectively.